Cannibal Holocaust, and it was because it was so damn good at what it does that I had to turn away.
But The Devil's Candy is on a whole other plane of existence. I just couldn't watch it anymore because it was so freaking bad. When the filmmakers think the simple image of an upside-down cross does their work for them, boy, you've got a problem.
That cuts right to the chase, really - they were phoning it in. Sure, the images created by the artist possessed by the devil were creepy, and sure, the psychologically/developmentally challenged killer does terrible things to children. But that's so tropey it hurts. On top of all that, "the devil made me do it" is not sinister - it actually elides how monstrous humans can be, and misses the whole point of the horror film-to show us the darkest parts of ourselves.
I think I'll just go back to watching people chop the heads off turtles, thanks.